The Resistivity Logs
The flow of electrical current is governed by Ohm's Law: E = Ir, where E is the potential difference (volts), I is the current (amperes), and r is the resistance (ohms).
The resistance of any material will vary with both its size and shape. The intrinsic property of the material to resist current flow is known as the resistivity and is related to resistance by: where A is the cross-sectional area and L is the length of the resistor. For the same volume of material, long, narrow resistors have higher resistances than short, wide ones. The units of resistivity can be seen from the equation to be ohm-meter2/meter, which is usually contracted to ohm-meter or ohm-m. Conductivity is the reciprocal of resistivity with units of mho/meters. R
The resistivities of sedimentary rocks are determined by the rock component types and their geometry. The common reservoir framework minerals of quartz, calcite and dolomite have resistivities in excess of 100 million ohm-meters, and so can be considered essentially as insulators. The same is true for hydrocarbons in the pore space. The fact that resistivities can be logged in sedimentary rocks is principally due to the conductivities associated with the formation water brine and the cation-exchange capacity of clay minerals within the shales.
At a temperature of 77 degrees Fahrenheit, drinking water ( 200 pprn NaCl ) has a resistivity of 26 ohm-m, for sea water ( 35,000 pprn NaCl ) the figure is